A. Field of the Invention
The present invention relates to a method and apparatus for detecting and responding to downed conductors in a power distribution system. More specifically, the present invention is a system for detecting a fault in a power conductor and in response thereto opening the circuit to the faulted power conductor. The system can be designed to provide either a single phase response or a three phase response to the detected fault.
B. Description of Related Art
The electrical power that runs today's modern society is typically generated in rural areas because of real estate restraints, environmental concerns and cooling water availability. In contrast, most load consumption is in large urban areas far removed from generating stations. A network of high voltage, three phase transmission lines operating at 230 kV to 765 kV connects rural generating stations with urban load centers. In addition, most power systems have a second, lower voltage subtransmission system used to transmit smaller blocks of power to regional load centers. Subtransmission voltages typically range from 4600 volts and 138,000 volts.
A distribution system transmits power from subtransmission facilities to power consumers, such as residential and commercial users. The distribution system operating at 4.6 kV to 35 kV typically provides power to the customer as a single phase line operating at 120 to 240 volts. The principal components of the overhead distribution system are commonly poles, hardware, an energized phase conductor (i.e., power conductor) and a neutral return conductor.
Distribution transformers of 5 kV, 15 kV, 25 kV, 35 kV class are connected between the energized phase conductor and the grounded neutral conductor. A medium voltage fuse connected between the distribution line and the medium voltage transformer, provides protection against short circuits in the transformer without affecting other consumers.
Branch line installations are the typical means for interconnecting the distribution systems. Single phase branch lines may also be individually equipped with devices which function like circuit breakers, called "reclosers". Reclosers help to ensure that a problem on any of the branch circuits does not affect the entire line. Branch lines usually have no alternate means of supply.
Main three phase lines, called feeders, originate at a substation and usually end at a normally opened switch. The normally opened switch is a tie point to another feeder to provide an alternate source of power. This flexibility increases the reliability of the system while allowing minimal inconvenience to consumers during system maintenance. Feeders serve a well defined area, with single phase branch lines tapped off the main line.
Faults which interrupt the flow of power to consumers are a recurring problem and a constant concern for utility companies. There are a number of different types of faults involving actual contact between power conductors. Single phase faults are the most likely to occur, and are therefore of the greatest concern to electrical utilities. For example, a phase conductor can break loose from its support and fall to the ground, causing a phase-to-ground fault. Fault current will flow from the substation transformer, through the faulted conductor and the earth where the conductor fell, and back to the substation via the parallel path consisting of the earth and the neutral. At first it seems that detection of such faults should be relatively straight forward because a short circuit involves only one phase and ground. This is not true.
Design of a reliable system to detect faults to earth is a task beyond the best technology available until the present invention. The quantity which makes this problem so difficult is that earth impedance at the point of fault can be large enough to limit fault current to below the level at which it can be detected. The majority of detection schemes in use on distribution systems today measure current magnitude as a problem indicator. This method is not foolproof because the current which flows as a result of a phase-to-ground fault is not predictable. Some phase-to-ground faults result in large current flow, while others cause virtually none. The following examples illustrate this unpredictability.
Consider a typical 7200 volt single phase distribution line under normal circumstances serving 100 residents where each resident has a load of 30 amperes at 240 volts. For simplicity, it is appropriate to assume this load to be all resistive. The total load current is: I=30 amperes.times.100 houses=3000 amperes at 240 volts. This current may be converted to a 7200 volt base by multiplying by the transformer ratio to obtain 100 amperes at 7200 volts.
Consider this same system operating with a short circuit. Several factors must be included to calculate the amount of current which will flow. The impedence of equipment between a generator and the station limits the current available at the source distribution station. A typical source impedance of 1 ohm is assumed, which would provide available fault current of 7200 amperes at the source station. Most faults do not occur right at the substation. Thus, a 1 ohm impedance for the distribution line should be included in any calculation of fault current. One ohm corresponds to approximately a mile of overhead line. The magnitude of the short circuit current between the phase conductor and the neutral is determined by dividing the source voltage by the sum of impedances between the source and the fault. The amount of resistance between the source and the fault would be the summation of three impedances, one ohm for the source, one ohm for the phase line and one ohm for the neutral. Thus, the short circuit current would be: ##EQU1##
Overcurrent devices that respond to current flow are used by electric utilities to detect and remove such faults. These devices isolate a circuit when high values of current flow through them. They must not isolate the circuit when normal values of load current are flowing. There is no difficulty sensing the short circuit, 2400 amperes, while at the same time not responding to load current, 100 amperes.
The problem of sensing downed conductors adds another dimension to this problem. Consider the same system as above, a station generating 7200 volts with a one ohm source resistance and one ohm line and neutral resistances but now include fault impedance typical of real life downed conductor situations. Tests have shown that a downed phase conductor may encounter approximately 75 ohms of impedance at the fault point. This is the resistance to current flow through the earth. Actual values of fault impedance may be lower but are frequently higher. The short circuit current value for this situation is: ##EQU2##
An over current device protecting this circuit must not operate for the load current of 100 amperes. With that constraint the need to operate for a 92 ampere fault poses a serious problem. This simplified example is indicative of the problem facing the electric utility industry. See generally, Downed Power Lines: Why They Can't Always Be Detected, IEEE Power Engineering Society, Feb. 22, 1989; and Detection of Downed Conductors on Utility Distribution Systems, IEEE Tutorial Course, Course Text 90 EH 0310-3-PWR.
Downed conductors can also pose a problem in three phase distribution systems. Most industrial and commercial customers use three phase circuits to provide electrical service to their facilities. The typical service contract between the utility and a customer does not guarantee three phase service largely because single pole fuse cutouts are used, which cutouts respond to a single phase overcurrent by interrupting current to a single phase. In recognition of this, industrial and commercial customers typically employ three phase switching and interrupting devices to provide a three phase response to a voltage imbalance. A severe imbalance can cause early failure of many types of electrical motors. Customers with smaller loads and equipment may not invest in the cost for under voltage protection and gamble that a single phase fault will not occur. Accordingly, there is a need for an economical way to provide a three phase response to fault on a single phase.